Integrated electrographic recording and developing stylus assembly

ABSTRACT

An improved electrographic recording stylus for electrographic recording systems includes a magnetic flux concentrating portion and a separate and distinct electrode portion connected to electronic circuitry furnishing voltage potential record pulses to the stylus for electronically charging toner that is in contact therewith.

United States Patent 11 1 1111 3,879,737

Lunde Apr. 22, 1975 INTEGRATED ELECTROGRAPHIC 3.611.018 10/1971 Ring 346/74 ES RECORDING AND DEVELOPING STYLUS 3.638.614 2/1972 Young ct al. 118/637 ASSEMBLY 3.816.840 6/1974 Kotz 346/74 ES [75] Inventor: George G. Lunde, White Bear Lake,

Minn Prlmary L.\'ummer--Bernard Konlck Assistant Examiner-Jay P. Lucas 1 Asslgneei Mllmesoia Mlllmg and Attorney, Agent, or FirmAlexander, Sell, Steldt &

Manufacturing Company, St. Paul. DeLaHum Minn.

[22] Filed: Apr. 8, 1974 57 ABSTRACT [21] Appl. No.: 458,720 1 An improved electrographic recording stylus for elec- [52] Cl. U 346/74 1 18/637. 355/3 DD trographic recording systems includes a magnetic flux 51] 1111. C1 00111 15/06- 601d 15/12 mmmmfing and a Separate and distinct {58] Field of Search 346/74 ES 4 74 EB trode portion connected to electronic circuitry fur- 346/74 s 74 Ex [39 118/637. 355/3 nishing voltage potential record pulses to the stylus for i 178/66 electronically charging toner that is in contact therewith.

[56] References Cited UNITED STATES PATENTS 7 Claims, 7 Drawing Figures 3.605.691 9/1971 Herman 118/637 PM :5 (aw/e04 aka 0/ T PATENTEDAPRZZIHYS 732,

SHEET 1 3 P0155 CONTROL c/ c u/ T all FIG. 2

FATENTl-immzzms SHEET 2 0F 3 FIG. 3

INTEGRATED ELECTROGRAPI-IIC RECORDING AND DEVELOPING STYLUS ASSEMBLY BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates in general to electrographic apparatus for the recording of toner images on a recording medium, and more specifically to a recording stylus that is employed in such apparatus.

2. Description of the Prior Art Prior art recording systems to which the present invention is related are disclosed in U.S. Pat. application Ser. No. 353,139, filed Apr. 20, l973 now US. Pat. No. 3816840, and US. Pat. application Ser. No. 415,623, filed Nov. 14, 1973, both of which are assigned to the assignee of the present invention. The recording systems disclosed in these prior applications include a cylindrical developer roll with permanent magnet sectors and an array of magnetically permeable recording styli that are disposed on the developer roll. The magnetic fields of the magnet sectors are concentrated in the recording styli, thus providing a relatively strong magnetic flux density at the outer ends of the styli, and the styli are selectively pulsed with voltage potentials by drive electronics to produce dot depositions of magnetic toner powder on a recording medium, which powder is presented to the recording mediumby the developer roll. There is a magnetic attractive force at the recording end of each stylus that is significantly higher than the surrounding magnetic field; consequently a dense concentration of powder at the outer end of each stylus results. The net effect on the quality of the recorded deposition is an increase of dot density and dot definition (resolution) with relatively low recording voltages.

All of the prior art styli constructions include the magnetic portion of the stylus as an integral, electrically continuous portion of the electrode portion, i.e., the magnetic portion provides both the magnetic control of the toner and the electrical control of the toner. To provide optimum toner deposition, the cross sectional area of such styli should be relatively large, the ultimate stylus having a solid iron wire electrode. With styli spacings of 50 to 200 styli/inch (representing the usual range for character or facsimile printing) the diameter of such styli can range from 0.010 inch down to 0.003 inch. In general, the spacing between two styli is about equal to the styli diameter. One problem with the use ofiron wire styli electrodes is that when they are wrapped around a cylindrical developer roll and brought out to a connecting point for the drive electronics, they form a partial shield that shunts the magnetic field emanating from the inner magnet sectors of the developer roll. This has the effect of weakening magnetic forces that cause the toner to tumble about the developer roll. Another disadvantage of using wire styli electrodes is the difficulty of connecting a large number of individual wires to the electronic drive circuitry.

In the aforementioned US. Pat. application Ser. No. 415,623, the styli are each entirely formed of magnetic material having a high magnetic permeability, and are each connected by a nonmagnetic, conductive lead of a low magnetic permeability to the drive electronic circuitry that provides record voltage potential pulses thereto. Such construction provides good recording performance because the low magnetically permeable leads connecting the styli with the drive circuitry do not shunt the magnetic field of the developer roll magnet sectors. However, when a dense array of a large number of styli are utilized, individual connections between each stylus and the drive circuitry are difficult to make and often shorting between styli results. Thus, methods and forms of construction become important in terms of cost as well as in terms of image quality and reliability.

In contrast to known prior art styli, an ideal styli array preferably should have styli electrode portions formed by a reliable technique such as etching a metal pattern on an insulating substrate as done in printed circuits for the electronics industry and the electrode portions should be capable of being connected to the drive electronics without interfering with toner flow or shorting to one another or to ground. In addition, the magnetic portions of the styli should provide a strong, well defined magnetic field at the printing end of the styli electrodes to concentrate and align toner particles in continuous chains from the styli to the surface of a recording medium, but should not shunt the magnetic field away from the styli or from the surface of the styli developer roll. Furthermore, the resulting styli array should present no obstructions on the surface of the developer roll to hinder the flow of toner. No prior art magnetic styli construction completely satisfies these ideal" styli array criteria.

The present invention provides an improved stylus construction that separates the magnetic stylus into two functionally separate constituents that are inseparable as far as the recording process is concerned. The magnetic portion of the stylus does not contribute to the electrical current flow to the recording toner, and the electrical portion of the stylus does not contribute to the magnetic control of the toner. This approach gives startling simplicity to the construction of the styli array, and high quality in the recording process results.

SUMMARY OF THE INVENTION The present invention resides in improved recording styli for electrographic recording systems having a recording electrode contacting one side of a recording medium, a developing means with a rotating inner magnet assembly, and an array of the recording styli spaced from said recording medium to define a recording region therebetween. The improved styli each include a first portion in which the magnet assembly of the developing means induces a magnetic flux density and a second portion that is connected to electronic circuitry furnishing voltage potential record pulses to the styli for electronically charging toner powder bridging the recording region to produce depositions of toner on the medium.

A plurality of the improved styli may be formed in a dense printing array by using a magnetically permeable crossbar positioned axially of the surface of the recording member to serve as the first portion of the styli, and overlying the crossbar are a plurality of spaced apart electrically conductive leads that serve as second portions of the styli. In a first preferred embodiment, the crossbar is electrically nonconductive, and each electrically conductive lead together with the crossbar forms an individual stylus. To facilitate fabrication of the styli, flexible printed circuitry may be employed to provide a plurality of conductive leads that are densely arranged in contact with the crossbar, and yet can be fanned out to standard connector spacing to readily connect to the electronic circuitry that furnishes the voltage record pulses. The flexible printed circuitry is bonded on the developer roll in a smooth arrangement to provide relatively no obstruction to the flow of toner as it tumbles around the circumference of the tube.

In a second preferred embodiment, the conductive leads are insulated from the crossbar to prevent electrical contact between the two portions of the styli. Thus, the crossbar may be formed from a wide variety of magnetic materials including those that are electrically conductive, such as iron, without the danger of electrical shorting between adjacent styli.

In both of the above described embodiments, the conductive leads are formed from a nonmagnetic material with relatively little shunting effect on the magnetic field induced in the crossbar of the styli. The crossbar serves as a path of high permeability for the magnetic flux lines of force emanating from the developing means magnet assembly. As a result, a high magnetic gradient exists at the outer edge of the styli crossbar to provide a strong attractive force for toner presented at the recording region. The magnetic toner is attracted to the styli and is concentrated in the form of toner chains that bridge between the recording medium and the styli cauusing dense quantities of toner to be deposited on the recording medium when a record pulse is applied to the styli.

The foregoing and other advantages of the present invention will appear from the following description. In the description reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration, and not of limitation, specific forms in which the invention may be embods ied. Such embodiments do not represent the full scope of the invention, but rather the invention may be employed in a variety of embodiments, and'reference is made to the claims herein for interpreting the breadth of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic side view'of an electrographic recording system incorporating the present invention and including a toner powder reservoir, a developer roll, a recording member, and a pulse control circuit;

FIG. 2 is a fragmentary perspective view of the developer roll of FIG. 1 illustrating an enlarged view ofa first embodiment of a printing array disposed thereon;

FIG. 3 is an enlarged view of flexible circuitry that forms a portion of the printing array of FIG. 2;

FIG. 4 is an enlarged, fragmentary view of a second embodiment of the printing array that mounts on the developer roll of FIG. 1; 7

FIG. 5 is an enlarged, fragmentary view of a modified embodimentof the printing array of FIG. 4;

FIG. 6 is an enlarged, fragmentary view of another modified embodiment of the printing array of FIG. 4; and

FIG. 7 is an enlarged, fragmentary view of yet another modified embodiment of the printing array of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 of the drawings, a schematic view is shown of a prior art electrographic recording system for whichthe present invention ,is particularly adapted to be used. This'prior art system includes a toner powder reservoir 1, a cylindrical developer roll 2, a rotatable recording member 3, and a pulse control electronic circuit 4. a

The developer roll 2 may be of the type disclosed in Anderson, US. Pat. No. 3,455,276, and has an inner magnet rotor 5 and an outer cylindrical shell 6 that is electrically nonconductive and is nonmagnetic. The term nonmagnetic with reference to a material is used herein to mean that the material acts nearly the same as air or a vacuum in the presence of a magnetic field. A magnetic circuit (field passing between North and South magnetic poles) is notaffected by the introduction of the material, and the material itself is not magnetized due to the magnetic field. The term magnetic material" is herein used to mean a material that easily passes magnetic flux (has a high permeability to the magnetic field) and therefore has a high susceptibility to being magnetized by a magnetic field. The magnet rotor 5 includes a cylindrical, magnet support core 7 and a plurality ofpermanent magnet sectors 8 arranged about the cylindrical surface of the support core 7, the sectors 8 defining an outer surface of alternate North and South magnetic poles. The developer roll 2 is mounted on an axle 9 with the support core 7 preferably rotated in a clockwise direction, whereas the outer shell 6 is spaced from the magnet rotor 5 and is preferably fixed in position.

Arranged on a line that runs axially of the exterior surface of the shell 6 are a plurality of individual, spaced apart recording styli 10 (only one of which is shown in FIG. 1). The styli 10 are normally utilized to form a printing matrix and the number of styli included in the matrix is dependent upon the printing application for which the matrix is to be used. In the case of a standard computer output line width of 136, 5 X 7 dot matrix characters, nearly 1,000 styli are employed, spaced at styli/inch. For more complex character fonts and simple graphic applications, styli spacings of IOO/inch toover 200/inch are required.

The recording member 3 mounts an axle 11 that is parallel to the developer roll axle 9, and is rotatably driven clockwise. The recording member 3 is preferably formed of an electronically conductive, cylindrical electrode 12 and an endless recording medium 13 that overlies the cylindrical surface of the electrode 12. The electrode 12 is positioned in a spaced, opposed relationship with the styli 10 to define a recording region 14 therebetween. The electrode 12 also serves as a support means for the recording medium 13, which is preferably formed from ananodized aluminum. I

The toner powder reservoir 1 is filled with a magnetically attractable, electronically conducting toner pow der 15, such as the powder disclosed in Nelson, US. Pat. No. 3,639,245. The reservoir 1 includes a doctor blade 16 for metering the toner 15 in an even layer onto the outer shell 6 of the developer roll 2. The toner 15 is held on the periphery of the outer shell 6 by the magnetic fields of the magnet sectors 8 and becomes arranged in the form of toner particlechains 17. The toner chains 17 are transported by theideveloperrollZ to the recording region 14. While in the recording region 14 certain of the toner chains-l7 bridge between the recording medium 13 and the styli l0, and portions of the toner chains 17 in contact with the recordingmedium 13 are deposited thereon.

Deposition of portions of'the toner chains 17 occurs when recording voltage pulses of sufficient magnitude are applied to various ones of the styli by the electronic recording pulse circuit 4. The record pulses produce a voltage potential between the cylindrical electrode l2 and the styli 10, which results in a force pattern of intelligence that exerts a force on the portions of the toner chains 17 contacting the medium 13. Such force opposes and overcomes the magnetic attraction of the magnet sectors 8 for portions of the toner chains 17 associated with the styli 10 to which the record pulses have been applied. This action is described in detail in US. Pat. application Ser. No. 353,139, filed Apr. 20, 1973, and naming Arthur R. Kotz as inventor, which application is herein incorporated by reference.

Although the present invention encompasses the basic electrographic recording process taught by Kotz, the present invention adds a new dimension to the teachings of Kotz in the form of the recording styli 10. The construction of the styli 10 is uniquely adapted for providing a large number of the styli 10 in a small area to form printing arrays that are reliable and readily fabricated. Thus, the styli 10 are particularly advantageous for forming styli printing arrays having a high density of printing styli.

Referring now to FIG. 2, a first preferred embodiment of the styli 10 of the present invention is shown forming a printing array 20 mounted on the developer roll 2. The printing array 20 includes a magnetically soft cross bar 21 having one lengthwise edge seated in a slot cut in the periphery of the developer roll outer shell 6, and an opposite edge that protrudes radially outward from the periphery of the shell 6. The bar 21 is formed of a magnetic material that is substantially electrically nonconductive. Each lengthwise side of the bar 21 is shielded by an encapsulating material in the form of two wedge shaped ramps 22. The bar 21 extends axially of the exterior surface of the outer shell 6 and a plurality of spaced apart nonmagnetic electrodes 23 are fixed on the ramps 22 to extend across and overlie the outer edge of the bar 21. A relatively electrically nonconductive material forms the bar 21 so that the electrodes 23 are electrically isolated from one another. Each electrode 23 together with the bar 21 defines one of the printing styli 10. For purposes of clarity the electrodes 23 are shown considerably enlarged in the drawings, and normally over 100 electrodes per inch would be employed in the array 20. The particular dimensions of the styli 10 are not critical and may vary depending upon the recording application. However, it has been found that for magnetically optimum conditions, the distance between the outer and inner lengthwise edges of the bar 21 should be ten times greater than the distance between the ramps 22.

Sheets of flexible printed circuitry 24, as indicated by FIG. 3, are preferably used to provide the plurality of electrodes 23 for the styli 10. The circuitry 24 is well known in the electronic art and is formed of a plurality of electrically conductive leads that are bound together by an electrically insulative material. One end of each sheet of the circuitry 24 is bonded to the bar 21 and the ramps 22 and the leads on the sheet provide electrodes 23 that are closely aligned together. The opposite end of the electrodes 23 of the sheets of circuitry 24 are fanned out to furnish a relatively wide separation therebetween for making a suitable connection with the record pulse circuitry 4. By use of the flexible circuitry 24 the possibility of a short occurring between the electrodes 23 is significantly diminished in comparison to that presented by former styli constructions.

The above described styli construction thus provides a two portion stylus including an electrically conducting portion and a magnetic portion. High quality recording of toner images is provided by the styli 10 because the bar 21 provides a path of high magnetic permeability for the magnetic flux of each of the magnet sectors 8. Accordingly, when a magnet sector 8 is aligned with the bar 21 a narrow area of high density magnetic flux along the length of the bar 21 extends out from the outer edge of the bar 21 in a perpendicular relationship with the recording medium 13 and back to the magnet rotor 5. The quality of the images provided by the styli 10 is in part dependent upon the radial length and positioning of the bar 21 for two reasons. First, the closer the inner edge of the bar 21 is brought to the magnet rotor 5, the greater the magnetic flux density in the bar 21 due to a narrowing of the low permeability air gap between the bar 21 and rotor 5. Secondly, if the protrusion of the outer edge of the bar 21 is increased, the differential between the flux extending from the outer edge of the bar 21 and the surrounding flux field emanating from the magnet rotor 5 also increases. The bar 21 furnishes a narrow planar surface in the recording region 14 having a strong magnetization for attracting and aligning the toner chains 17 in the region 14. When the toner chains 17 approach the printing array 20, they are attracted to and concentrated at the outer end of the bar 21, whereas the areas on either side of the bar 21 are substantially void of toner. The electrodes 23 may be conductive along their entire length but the printing area of a given styli 10 is determined by the intersection of the electrode 23 for the given styli 10 and the magnetic lines of force extending from the bar 21 adjacent to the given styli 10.

It is not essential to the present invention that the electrodes 23 are in electrical contact with the bar 21 since the bar 21 functions only as a means of supplying a strong magnetic field to the electrodes 23. This is evidenced by a second embodiment of the present invention represented by printing styli 28, as shown in FIG. 4, forming a printing array 29. The array 29 is similar to the array 20 in that it includes a bar 30 formed of a magnetic material positioned on the shell 6 in the same manner as the bar 21, and is bordered on both lengthwise sides by wedge shaped ramps 31. However, the array 29 differs from the array 20 in that overlying and bonded to the exterior of the ramps 31 and the outer edge of the bar 30 is a flexible printed circuit that includes both an electrically insulating support layer 32 and a copper layer etched to furnish an appropriate pattern of electrodes 34 that extend across the outer edge of the bar 30. Consequently, the bar 30 is electrically insulated from the electrodes 34 by the layer 32, and as a result the bar 30 may be formed from a material that is electrically conductive, such as iron. Cover sheets 35 of insulative material, as shown in FIG. 5, may be bonded to the ramp surfaces of the electrodes 34 to provide a smoother surface for the toner chains to flow over, but such sheets are merely a matter of preference.

The above described embodiments are not exhaustive in describing the present invention for a number of variations of the above embodiments would readily be perceived to fall within the inventive spirit of the present invention. For example, referring now to FIG. 6, in which a modified form of the second embodiment is illustrated, it may be seen that it is not essential to the present invention that the electrodes 34 bridge across the outer edge of the bar 30. Instead the bridging portion of the electrodes 34 and the end portion of the cover sheet 35, if present, may be removed to leave only end portions 36 of the electrodes 34 exposed. Alsoyas shown in FIG. 7, the portion of the insulating layer 32 covering the outer end of the bar 30 may be ground away to expose the outer end of the bar 30. An advantage of this latter configuration is that the bar 30 can be electrically grounded when desired to aid in erasing previously recorded material on the recording medium 13.

Other variations of the present invention may include shaping of the bars 21, 30 to change the shape of the magnetic field near the exposed ends of the electrodes 23, 34. The bars 21, 30 can also have slots cut to form teeth which are each located directly under the exposed ends of the electrodes 23, 34. Moreover, the top of the bars 21, 30 can be cut at an angle to form a sharp edge which is located in a line under the exposed ends of the electrodes 23, 34. Either of these modifications tend to increase the strength of the magnetic field at the exposed portion of the electrodes 23, 34, but also make construction of the styli arrays 20, 29 more difficult.

The following non-limiting example is now provided.

EXAMPLE 1 Referring to FIG. 5, the iron bar 30, 0.010 inches by 0.094 inches (after machining) by 9 inches long, was made from 1,010 steel shim stock and inserted into a 0.031 inch deep slot cut into the developer roll 2. The bar 30 was then bonded onto the roll 2 using a two part room cure epoxy that was then machined to form the ramps 31 shown. The height of the ramps 31 from the developer roll outside diameter was 0.063 inches, and the angle of the two ramps was 45 from the outer end of the bar 30. The top of the bar 30 was ground flat within 0.001 inches for the 9 inch length of the top of the ramp. The array of styli 28 was formed using flexible printed circuit material of 1 ounce (approximately 1.5 mil thick) electrolytic copper bonded to a 1 mil thick polyimid substrate with an epoxy thermoset adhesive. The copper was etched to form electrodes 34 each 7 mils wide with the electrodes 34 on 13.5 mil centers. The electrodes 34 were fanned out to 50 mil centerline spacing to mate with a printed circuit board or connector. Six such flexible circuits were combined to form an assembly providing a styli array of 576 styli with a spacing of 74 styli/inch. The array of styli 28 was then bonded to the ramps 31 and bar 30, with an intimate bond achieved by using a press with an elastic insert to form the flexible circuits to the shape of the ramps 31. The remainder of the circuit was wrapped around the developer roll and cemented down with epoxy adhesive.

What is claimed is:

1. In an electrographic recording system that includes a recording electrode electrically connected to one side of a recording medium, a toner developing means having an inner rotating magnet assembly and an outer shell, a plurality of recording styli fixed on the outer shell of said developing means and spaced from the opposite side of said recording medium to define a recording region therebetween that is bridged by toner powder transported on said developing means, and electronic circuitry for providing voltage potential record pulses to said styli, each recording stylus comprising the combination of:

a magnetic portion having one end positioned radially outward from said outer shell and formed of a magnetic material such that magnetic flux provided by the magnetic field of said inner magnet assembly is concentrated at the outer end of said stylus to cause a concentration thereat of the toner; and

a nonmagnetic electrode portion positioned near said end of said magnetic portion and connected to said electronic circuitry in order that voltage potential record pulses may be applied to said electrode portion to electronically charge the toner powder bridging the recording region and produce deposition of toner on said recording medium in accordance with applied record pulses.

2. In a recording system as recited in claim 1 wherein the magnetic portion of each of said recording styli is formed from an electrically nonconductive material and said electrode portion of each of said styli overlies and is in physical contact with said magnetic portion.

3. In a recording system as recited in 'claim 1 wherein the magnetic portion of each of said styli is electrically insulated from said electrode portion.

4. In a recording system as recited in claim 3 wherein said magnetic portion is electrically conductive.

5. In a recording system as recited in claim 1 wherein an electrically insulating layer overlies the end of said magnetic portion and said electrode portion overlies said insulative layer.

6. In an electrographic recording system that includes a recording electrode electrically contacting one side of a recording medium, a toner developing means having an inner rotating magnet assembly and an outer shell, a plurality of recording styli fixed on the outer shell of said developing means and spaced from the opposite side of said recording medium to define a recording region therebetween that is bridged by toner powder transported on said developing means, and electronic circuitry for providing voltage potential record pulses to the styli, said recording styli comprising the combination of:

at least one magnetic portion having an end positioned radially outward from said outer shell such that the magnetic flux provided by the magnetic field of said inner magnet assembly is concentrated at the outer end of said styli; and

a plurality of spaced apart nonmagnetic electrode portions positioned near said end of said magnetic portions and connected to said electronic circuitry in order that voltage potential record pulses may be applied to said electrode portions to electronically charge the toner bridging the recording region to produce deposition of toner on said recording medium in accordance with applied record pulses.

7. In an apparatus for the electrographic recording of toner powder images on a recording medium and having a stylus means and electrode means in spaced opposed relationship to provide a recording region therebetween, a developing means having an outer shell and an inner rotor member that rotates with respect to said outer shell and includes a plurality of alternating North and South magnetic poles on its circumference, toner supply means for supplying electronically conductive,

9 10 magnetically attractable toner to said developing stylus means; and means to be transported about the periphery of the dean electrically conductive nonmagnetic portion posiveloping means outer shell and electronic circuitry for tioned with respect to said magnetic portion to inproviding voltage potential pulses to said stylus means, tersect the magnetic lines of flux at the end of said which stylus means is mounted on the outer shell of stylus means, which conductive portion has record said developing means and comprises the combination voltage potential pulses applied thereto to elecof: tronically charge toner powder that is in electrical a magnetic portion aligned with the rotor member of contact with said conductive portion and said resaid developing means such that the magnetic flux cording medium at the time of the application of provided by said North and South poles of said desaid pulses.

veloping means is concentrated at one end of said 

1. In an electrographic recording system that includes a recording electrode electrically connected to one side of a recording medium, a toner developing means having an inner rotating magnet assembly and an outer shell, a plurality of recording styli fixed on the outer shell of said developing means and spaced from the opposite side of said recording medium to define a recording region therebetween that is bridged by toner powder transported on said developing means, and electronic circuitry for providing voltage potential record pulses to said styli, each recording stylus comprising the combination of: a magnetic portion having one end positioned radially outward from said outer shell and formed of a magnetic material such that magnetic flux provided by the magnetic field of said inner magnet assembly is concentrated at the outer end of said stylus to cause a concentration thereat of the toner; and a nonmagnetic electrode portion positioned near said end of said magnetic portion and connected to said electronic circuitry in order that voltage potential record pulses may be applied to said electrode portion to electronically charge the toner powder bridging the recording region and produce deposition of toner on said recording medium in accordance with applied record pulses.
 1. In an electrographic recording system that includes a recording electrode electrically connected to one side of a recording medium, a toner developing means having an inner rotating magnet assembly and an outer shell, a plurality of recording styli fixed on the outer shell of said developing means and spaced from the opposite side of said recording medium to define a recording region therebetween that is bridged by toner powder transported on said developing means, and electronic circuitry for providing voltage potential record pulses to said styli, each recording stylus comprising the combination of: a magnetic portion having one end positioned radially outward from said outer shell and formed of a magnetic material such that magnetic flux provided by the magnetic field of said inner magnet assembly is concentrated at the outer end of said stylus to cause a concentration thereat of the toner; and a nonmagnetic electrode portion positioned near said end of said magnetic portion and connected to said electronic circuitry in order that voltage potential record pulses may be applied to said electrode portion to electronically charge the toner powder bridging the recording region and produce deposition of toner on said recording medium in accordance with applied record pulses.
 2. In a recording system as recited in claim 1 wherein the magnetic portion of each of said recording styli is formed from an electrically nonconductive material and said electrode portion of each of said styli overlies and is in physical contact with said magnetic portion.
 3. In a recording system as recited in claim 1 wherein the magnetic portion of each of said styli is electrically insulated from said electrode portion.
 4. In a recording system as recited in claim 3 wherein said magnetic portion is electrically conductive.
 5. In a recording system as recited in claim 1 wherein an electrically insulating layer overlies the end of said magnetic portion and said electrode portion overlies said insulative layer.
 6. In an electrographic recording system that includes a recording electrode electrically contacting one side of a recording medium, a toner developing means having an inner rotating magnet assembly and an outer shell, a plurality of recording styli fixed on the outer shell of said developing means and spaced from the opposite side of said recording medium to define a recording region therebetween that is bridged by toner powder transported on said developing means, and electronic circuitry for providing voltage potential record pulses to the styli, said recording styli comprising the combination of: at least one magnetic portion having an end positioned radially outward from said outer shell such that the magnetic flux provided by the magnetic field of said inner magnet assembly is concentrated at the outer end of said styli; and a plurality of spaced apart nonmagnetic electrode portions positioned near Said end of said magnetic portions and connected to said electronic circuitry in order that voltage potential record pulses may be applied to said electrode portions to electronically charge the toner bridging the recording region to produce deposition of toner on said recording medium in accordance with applied record pulses. 